As a continuous casting method for steel, a method of injecting molten steel into a mold (casting mold) with a submerged nozzle having two discharge ports has been widely employed. The molten steel discharged from the submerged nozzle unavoidably contains bubbles, non-metallic particles, and the like mixed therein. Representative examples of the bubbles include argon gas bubbles. Argon is blown into the molten steel in the process of refining, such as VOD and AOD, used as a seal gas for a tundish, or intentionally added to the molten steel flow channel for preventing clogging of the nozzle, but is substantially not dissolved in the molten steel, and thus tend to mix in the mold as bubbles. The non-metallic particles mainly include a part of such materials as a slag for refining, a deoxidation product formed in the refining process, a refractory as a constitutional material of a ladle and a tundish, and powder existing on a molten steel surface in a tundish, which are entrained into the molten steel, and flow into the mold along with the molten steel through the submerged nozzle. Separately, mold powder is added to the surface of the molten steel in the mold. The mold powder generally floats on the molten steel surface and covers the surface of the molten steel, and has functions, such as lubrication between a cast piece and the mold, heat retention, and antioxidation, and also a function trapping non-metallic particles emerging on the molten steel surface.
The bubbles and the non-metallic particles flowing into the molten steel in the mold float in the mold along with the flow of the molten steel, and those having a relatively large size tend to emerge near the molten steel surface, and may be entrained in some cases into the solidification shell (i.e., the surface layer portion of the cast piece) formed in the initial stage. The mold powder on the molten steel surface may also be entrained in some cases into the solidification shell in the initial stage. In the following description, the bubbles and the substances, such as the non-metallic particles and the mold powder, in the molten steel entrained into the solidification shell, and the substances having been entrained into the solidification shell are referred to as “foreign matters”. The incorporation of foreign matters to the solidification shell may be a factor forming a defect (flaw) on the surface of the steel sheet obtained through the process including hot rolling and cold rolling.
In the continuous casting of steel, electro-magnetic stirrer (EMS) is effective as a measure for suppressing the incorporation of foreign matters to the solidification shell, and has been widely used (see, for example, PTL 1). It has been empirically confirmed that foreign matters can be prevented from being entrained into the solidification shell by making the molten steel in the vicinity of the solidification shell to flow forcedly.
In the case where the temperature of the molten steel surface in the mold is decreased, it is considered that the initial solidification shell may be formed with an uneven thickness due to the influence of the heat removal from the molten steel surface. The uneven initial solidification shell descends along the surface of the mold while exhibiting a craw-like cross section, and becomes a factor increasing the entrainment of foreign matters into the solidification shell. Accordingly, the retention of the temperature of the molten steel surface to a high temperature is also effective for suppressing the entrainment of foreign matters into the solidification shell.
PTL 2 describes that the discharge angle of the submerged nozzle is in a range of from 5 to 30 degrees upward from the horizontal direction (PTL 2, paragraph 0013). In the case where the casting rate is as small as 0.9 m/min or less, the inverse flow directed to the submerged nozzle from the short edge is small (ditto, paragraph 0021), and thus the temperature of the molten steel in the vicinity of the meniscus cannot be retained to a high temperature by the ordinary feed of the molten steel. The problem is then solved by directing the discharge angle of the nozzle upward from the horizontal direction, so as to facilitate the supply of heat to the meniscus (ditto, paragraph 0022). It is stated that in the case where the molten steel is discharged upward from the submerged nozzle, a flow thereof directed directly to the meniscus is formed, by which the molten steel having not been cooled with the mold is fed to the meniscus, so as to increase the temperature of the meniscus (ditto, paragraph 0023).
PTL 2 also describes a method of retaining the temperature of the molten steel in the vicinity of the meniscus to a high temperature by performing electro-magnetic stirring in the same direction on the long edge surfaces on both sides to increase or decrease the velocity of the inverse flow from the short edge, in the case where the casting rate is as large as approximately from 0.9 to 1.3 m/min or approximately 1.3 m/min or more (ditto, paragraphs 0025 to 0029). In this case, it is taught that the discharge angle may be relatively small (ditto, paragraph 0029), and 5° upward is employed in the example (ditto, Table 2). With a discharge angle of 5° upward, the discharged flow from the submerged nozzle is directed to the short edge surface, and the inverse flow from the short edge flows to the molten steel surface.